Before asking your teacher questions about how to do the lab, please read carefully, twice, the entire "Day One" document.

Special Note: I will deduct 10% off your grade for playing with the model sets; as in "Look Mr. ChemTeam, at this weird structure I made" or if I see you making a structure not in the assignment. On the second occurrence, I will zero your grade.

Objective

1) Using a model building kit, construct models of a variety of simple covalent molecules.
2) Draw Lewis structures and/or structural formulas of selected models.
3) Draw all the isomers of selected formulas.

Brief Overview

In 1874, J. H. van 't Hoff (1901 Nobel Prize in Chemistry) was the first to suggest that molecules have a three-dimensional structure. He used this idea to explain several previously puzzling facts about chemical compounds.

In this lab, we will use a kit to model the 3D structure of a number of molecules, including several that van 't Hoff focused on. After building the molecular models, you will draw them on paper in a manner intended to represent the 3D appearence. You will also draw Lewis structures for some of the molecules as well as isomeric structures for those formulas that have them.

Safety

The parts of the kits are not toys. Use them only as instructed and only for the purposes of the lab.

Equipment List

One molecular model kit

Procedure

Part I. Preliminary Steps

1) Examine the front cover of the model building kit. At the bottom, it tells you the colors for each element. When you open the kit, there is one main component missing. These are small, white connectors which you will not be using. Also, there may be two shades of green for the chlorine atoms. This has no chemical meaning. It is just that the kits were bought at different times and the green pigment varied from lot to lot.

2) Examine the various atom models and note the number of holes in each. When you make a model, you MUST fill up each hole with a bond. For example, black represents carbon and each carbon sphere has four holes. To make correct structures, you MUST fill all four. There is one exception to this. The pale blue atom (representing nitrogen) has four holes. Please use only three. By the way, carbon is black and nitrogen is blue. Those of you who use a blue sphere for carbon have made a mistake and, most likely, you made the mistake because you didn't read (and remember) this particular section.

3) Examine the light gray connectors. The short, thick ones are to be used to show single bonds. The longer, bendable ones are for double and triple bonds. You need to use two for double bonds and three for triple bonds. Do not mix long and short connectors between bond types. In day four, there will be a situation where you need to use the thinner connectors for single bonds.

4) Make the following molecules: H₂, O₂ and N₂. These three demonstrate the use of single, double and triple bonds. Please do not click the link until you have built the models.
A picture of these three models.

5a) Now, make H₂O and NH₃. Both of these molecules use only single bonds. Please do not click the link until you have built the models.
A picture of these two models.

5b) Now, make CO₂ and HCN. The first uses two double bonds and the second uses one triple bond and one single bond. Please do not click the link until you have built the models.
A picture of these two models.

Data Table Comment: more than likely, many of you will start drawing structures of H₂ and the others just above. This is a mistake due, most likely, to your failure to read through the entire document (and remember the important parts) before beginning work. Before starting to draw structures, please make sure you review all the instructions below concerning how to make the data table.

Part II. Experimental Steps

6) Following the list below, continue to build models. There are not enough atoms to build all the molecules discussed below, so build some, then break them down, then build more.

a) Alkane (7 structural, 3 Lewis) is the category name for a set of compounds which contain carbon and hydrogen and ONLY single bonds. An alkane has the general formula of C_nH_{2n + 2}. Build the following alkanes:

1. methane, CH₄ A picture of methane
2. ethane, C₂H₆ A picture of ethane
3. propane, C₃H₈ A picture of propane
4. butane, C₄H₁₀
5. pentane, C₅H₁₂ Butane and pentane in one picture
6. hexane, C₆H₁₄
7. heptane, C₇H₁₆ Hexane and heptane in one picture

Special Note: there are TWO (or more) different ways to make some structures, starting with the C₄H₁₀ formula. Make only the structure where the carbons are connected in a line. Do not make any structures that have the carbons branching. See the discussion below for more on this point.

b) Alkene (3 structural, 2 Lewis) is the category name for a set of compounds which contain carbon and hydrogen, ONE double bond and the rest single bonds. An alkene has the general formula of C_nH_2n. Build the following alkenes:

8. ethene (also called ethylene), C₂H₄ A picture of ethene
9. propene (also called propylene), C₃H₆ A picture of propene
10. butene (also called butylene), C₄H₈ A picture of butene

Special Note: Butene can have the double bond in two different locations. For your answer, please use only the structure where the double bond is between the first and second carbons.

c) Alkyne (3 structural, 2 Lewis) is the category name for a set of compounds which contain carbon and hydrogen, ONE triple bond and the rest single bonds. An alkyne has the general formula of C_nH_{2n - 2}. Pronounce alkyne to rhyme with "nine." Build the following alkynes:

11. ethyne (also called acetylene), C₂H₂ A picture of ethyne
12. propyne, C₃H₄ A picture of propyne
13. butyne, C₄H₆ A picture of butyne

Special Note: Butyne can have the triple bond in two different locations. For your answer, please use on the structure where the triple bond is between the first and second carbons.

Part III. Clean-up Steps

7) Unconnect all bonds from the atom models and replace in the box. Inspect around your table top and on the floor for lost atoms or bonds.

8) With any remaining time in the class period,

Data Table

Molecular Formula	Structural Formula	Lewis Structure
CH4
C2H6
C3H8
C4H10		not required (read instructions below)
etc.	etc.	etc.

Please read carefully all of the instructions below. Let's repeat that: please read carefully all of the instructions below. Also, please do this BEFORE you come up to the teacher, asking questions about what to do.

Data Table Instructions: The above data table should cover all 13 of the assigned substances in Part II, Experimental Steps, above (13 structural, 7 Lewis). (You need not draw any structures from Part I, Preliminary Steps.) The first column of the data table need only be wide enough for the molecular formula. I recommend against making the three columns each 1/3 of the paper.

1) There should be only one structural formula for each molecular formula. Please use lines and wedges as explained below.
2) Do Lewis structures as well for molecules with three or less carbons. Lewis structures use dots only, no lines whatsoever. That means with 4 or more carbons, only show a structural drawing in addition to the molecular formula. In other words, do not do Lewis structures for the compounds with 4 or more carbons.
3) In your drawings, do not show any hydrogens on substances with 4 or more carbons, except for the hydrogen on an -OH. This applies to answer the discussion question as well as to the data tables.

It is best to make a rough draft of the table first, then make a final draft. Students have a tendancy to allow for too little space to draw structures, then they cram them into the too small space and then say "Whaaaaat?" when the teacher deducts points. Your shock and outrage at being so treated will not deter me in my professional judgment of your data table.

Please draw your structural formulas in the same style as the CH₄ is done. The meaning of the three kinds of bond symbols in the above data table example:

1) the solid wedge bond means the hydrogen is coming out of the plane of the paper.
2) the dashed line (sometimes a wedge) bond means the bond is going away from you, behind the plane of the paper.
3) the solid line bond means the bond is in the plane of the paper.

Always have as many atoms as possible in the plane of the paper. For example, C₂H₂ can have all 4 atoms in the plane, as if you were above the molecule, looking down. C₂H₄ can have all 6 atoms in the plane. Once again, maximize the number of atoms in the plane.

By the way, the plane you should use to put atoms in is the one in front of you and parallel to you. Do not select a plant that would intersect your body. In other words, one at an angle (like 90° or perpendicular) to your body.

Also, the carbon atoms should be oriented left to right. For example, consider a four-carbon chain. Hold the molecule so the atoms from a sawtooth pattern running left and right, as opposed to the four carbon atoms being viewed up and down. Another way to say it: the carbon atoms should be viewed "spread out" as opposed to "on top of each other."

In your drawing, try to approximate the angles between atoms, as revealed in the model you've made. Once again, in C₂H₂, you would draw the hydrogens at 120° to the placement of the double bond.

As much as possible, make all the carbons in a chain to be co-planar, with only hydrogens (or chlorine or whatever) going behind or in front of the plane. In addition, make one hydrogen on each terminal carbon be co-planar with the carbon chain. When oxygen or nitrogen is involved, try to make them co-planar to the carbons.

Here is a link which has a discussion (half-way down) about dashed, wedge and line bonds.

Special Data Table Notes: (1) Do not draw bonds in the curved manner as seen in the picture of the O₂ molecule. Draw the double bond (and triple bond) using straight lines only. (2) Do not draw the element symbols with circles around them. Do not draw only circles to represent atoms. What you should draw are the letters of the elements, as in the CH₄ structure in the data table example.

Discussion

Structural Isomerism

The general definition of isomer is: two (or more) different chemical substances that have the same molecular formula. The key point is that the substances are DIFFERENT chemicals, but have the same number of each kind of atom.

There are a number of different ways to make isomers; the simplest of which is to have different connectivity. Connectivity refers to how the atoms (carbon, nitrogen, oxygen, hydrogen, etc.) in a molecule are connected to one another.

For example, consider the formula C₄H₁₀. There are TWO ways to connect the four carbons. Can you figure them out before looking at the picture?

The two correct isomers of C₄H₁₀

By the way, there are only two. Make the top structure in the picture, again if necessary, and then twist it to some other arrangement. Maybe like the one to the right? Is it a different substance than the top one in the other picture? If you say yes, what's to stop me from just twisting it right back to the original way? You cannot bend and twist a molecule to get a different connectivity. You must remove one (or more) carbons and put them SOMEWHERE ELSE (than from where they were removed) in the molecule.

Here's your discussion assignment: draw all the isomers for C₅H₁₂ (there are three), C₆H₁₄ (there are five), and C₇H₁₆ (there are nine). Want more? NOOOOOOOOOOO!!!! cries the poor, befuddled chemistry student!! OK, but if you did want more, there are 18 isomers of the formula C₈H₁₈, but you don't have to do them, just the C-4, C-5, C-6, and C-7.

There is no fast and easy way to construct possible isomers. The only way is through trial-and-error. However, do not be too quick to judge that you have, say, the nine isomers of C₇H₁₆. If one structure can be bent or twisted to exactly overlap with another, then the two models are of the same substance.

There is, however, a systematic way to approach the trial-and-error. First, make the straight chain molecule, as you did above for the data table. Now, remove ONE carbon and move it to a different location. After you've exhausted the possibilities, then move TWO carbons. However, keep in mind that it gets a bit more complex. Two carbons can be moved as two one-carbon fragments or one two-carbon fragment. Keep on exhausting all the possibilities.

However, one warning: if you get more isomers than I did, who do you think stands the better chance of being wrong?

Do not do them in the data table format above. I suggest a title like "The nine isomers of C₇H₁₆" and then the structures neatly done on the page.

Do not draw the C-4, C-5, C-6, C-7 isomers using wedge and dashed bonds. Draw them flat, using only solid lines for bonds. You do not need to draw them in a sawtooth pattern, as the C-4 example is done in the data table.

Go to the Day Two Instructions for this lab.

Here is another person's concept of a model building lab. Check it out for ideas to help you. Warning: I recommend you print it out.